Interconnect structure for semiconductor devices

ABSTRACT

A method of forming a semiconductor device includes: forming a first conductive feature in a first dielectric layer disposed over a substrate; forming a second dielectric layer over the first dielectric layer; etching the second dielectric layer using a patterned mask layer to form an opening in the second dielectric layer, where the opening exposes the first conductive feature; performing an ashing process to remove the patterned mask layer after the etching; wet cleaning the opening after the ashing process, where the wet cleaning enlarges a bottom portion of the opening; and filling the opening with a first electrically conductive material.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.17/232,465, filed Apr. 16, 2021 and entitled “Interconnect Structuresfor Semiconductor Devices,” which claims the benefit of the U.S.Provisional Application No. 63/085,217, filed Sep. 30, 2020 and entitled“Novel H2/Ar Plasma Treatment to Enlarge Co Recess Profile Beyond 3 nmNode,” which applications are hereby incorporated herein by reference.

BACKGROUND

High-density integrated circuits, such as Very Large Scale Integration(VLSI) circuits, are typically formed with multiple metal interconnectsto serve as three-dimensional wiring line structures. The purpose of themultiple interconnects is to properly link densely packed devicestogether. With increasing levels of integration, a parasitic capacitanceeffect between the metal interconnects, which leads to RC delay andcross-talk, increases correspondingly. In order to reduce the parasiticcapacitance and increase the conduction speed between the metalinterconnections, low-k dielectric materials are commonly employed toform Inter-Layer Dielectric (ILD) layers and Inter-Metal Dielectric(IMD) layers.

Metal lines and vias are formed in the IMD layers. A formation processmay include forming an etch stop layer over first conductive features,and forming a low-k dielectric layer over the etch stop layer. The low-kdielectric layer and the etch stop layer are patterned to form a trenchand a via opening. The trench and the via opening are then filled with aconductive material, followed by a planarization process to removeexcess conductive material, so that a metal line and a via are formed.

As feature sizes continue to shrink in advanced semiconductormanufacturing process, new challenges arise for semiconductormanufacturing. There is a need in the art for structures and methods forinterconnect structures that are suitable for advanced semiconductormanufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-3, 4A, 4B, and 5-8 illustrate cross-sectional views of asemiconductor device at various stages of manufacturing, in accordancewith an embodiment.

FIG. 9 illustrates a flow chart of a method of forming a semiconductordevice, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly. Throughout the description herein, unless otherwisespecified, the same or similar reference numerals in different figuresrefer to the same or similar element formed by a same or similarformation method using a same or similar material(s).

An interconnect structure of a semiconductor device and the method offorming the same are provided in accordance with some embodiments. Insome embodiments, an anisotropic plasma etching process is performed toform an opening in a second dielectric layer, where the opening exposesa conductive feature in a first dielectric layer underlying the seconddielectric layer. After the opening is formed, the conductive feature istreated with a plasma process, wherein the plasma process converts anupper portion of the conductive feature into a first material. After theplasma process, a multi-step wet cleaning process is performed usingdeionized water (DIW) to clean the opening. The DIW dissolves and removethe first material, thereby enlarging a bottom portion of the opening. Aconductive material is then formed to fill the opening to form a via.The enlarged bottom portion of the via formed may catch CMP slurryseeping down through cracks between the via and the second dielectriclayer, and may reduce recessing of the upper surface of the conductivefeature due to etching of the conductive feature by the slurry.

FIGS. 1-3, 4A, 4B, and 5-8 illustrate cross-sectional views of asemiconductor device 100 at various stages of manufacturing, inaccordance with an embodiment. The semiconductor device 100 may be adevice wafer including active devices (e.g., transistors, diodes, or thelike) and/or passive devices (e.g., capacitors, inductors, resistors, orthe like). In some embodiments, the semiconductor device 100 is aninterposer wafer, which may or may not include active devices and/orpassive devices. In accordance with yet another embodiment of thepresent disclosure, the semiconductor device 100 is a package substratestrip, which may be package substrates with cores therein or may becore-less package substrates. In subsequent discussion, a device waferis used as an example of the semiconductor device 100. The teaching ofthe present disclosure may also be applied to interposer wafers, packagesubstrates, or other semiconductor structures, as skilled artisansreadily appreciate.

As illustrated in FIG. 1 , the semiconductor device 100 includes asemiconductor substrate 101 (also referred to as a substrate). Thesemiconductor substrate 101 may include a semiconductor material, suchas silicon, doped or undoped, or an active layer of asemiconductor-on-insulator (SOI) substrate. The semiconductor substrate101 may include other semiconductor materials, such as germanium; acompound semiconductor including silicon carbide, gallium arsenic,gallium phosphide, gallium nitride, indium phosphide, indium arsenide,and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP,AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof.Other substrates, such as multi-layered or gradient substrates, may alsobe used.

Electrical components, such as transistors, resistors, capacitors,inductors, diodes, or the like, are formed in or on the substrate 101,e.g., in the front-end-of-line (FEOL) processing of semiconductormanufacturing. In the example of FIG. 1 , semiconductor fins 103 (alsoreferred to as fins) are formed protruding above the substrate 101.Isolation regions 105, such as shallow-trench isolation (STI) regions,are formed between or around the semiconductor fins 103. Gate structures109 are formed over the semiconductor fins 103. Each of the gatestructures 109 may be, e.g., a metal gate structure that includes a gateelectrode, work function layer(s) around the gate electrode, and a gatedielectric layer around the work function layer(s). Details of the gatestructures 109 are known to one skilled in the art, thus not repeatedhere. Gate spacers 111 are formed along sidewalls of the gate structures109. In the example of FIG. 1 , a cap layer 115 (e.g., a layer oftungsten) is formed over the gate structure 109, and a mask layer 116(e.g., titanium nitride) is formed over the cap layer 115. Source/drainregions 107, such as epitaxial source/drain regions, are formed onopposing sides of the gate structure 109.

After the electrical components (e.g., FinFETs) are formed, anInter-Layer Dielectric (ILD) layer 117 is formed over the semiconductorsubstrate 101 and around the gate structures 109. The ILD layer 117 mayfill spaces between the gate structures 109. In accordance with someembodiments, the ILD layer 117 comprises silicon oxide, phosphosilicateglass (PSG), borosilicate glass (BSG), boron-doped phosphosilicate glass(BPSG), fluorine-doped silicate glass (FSG), or the like. The ILD layer117 may be formed using spin coating, Flowable Chemical Vapor Deposition(FCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Low PressureChemical Vapor Deposition (LPCVD), or the like.

Still referring to FIG. 1 , contact plugs 113, such as source/draincontacts, are formed in the ILD layer 117, which contact plugs 113electrically couple the source/drain regions 107 to subsequently formedconductive features such as metal lines, vias, and conductive pillarsover the source/drain regions 107. Note that in the present disclosure,unless otherwise specified, a conductive feature refers to anelectrically conductive feature, and a conductive material refers to anelectrical conductive material. In an example embodiment, the contactplugs 113 are formed of a conductive material such as cobalt, althoughother suitable conductive materials, such as tungsten, aluminum, copper,titanium, tantalum, titanium nitride, tantalum nitride, alloys thereof,and/or multi-layers thereof, may also be used. The formation of thecontact plugs 113 may include forming contact openings in the ILD layer117 to expose the source/drain region 107, forming one or moreconductive material(s) in the contact openings, and performing aplanarization process, such as a Chemical Mechanical Polish (CMP), tolevel the top surface of the contact plugs 113 with the top surface ofthe ILD layer 117. Similar processing may be performed to form gatecontacts (not illustrated) over and electrically coupled to the gatestructures 109. In some embodiments, the gate contacts are formed byforming openings that extend through the mask layer 116 and the caplayer 115 to expose the gate electrode of the gate structure 109, thenfilling the openings with a conductive material, such as tungsten. Thegate contacts may not be in the same cross-section with the source/draincontacts 113, thus not illustrated in FIG. 1 .

Next, an etch stop layer (ESL) 119 is formed over the ILD layer 117. Insome embodiments, the ESL 119 comprises silicon nitride formed by PECVD,although other dielectric materials such as nitride, carbide,combinations thereof, or the like, and alternative techniques of formingthe ESL 119, such as LPCVD, PVD, or the like, could alternatively beused. In some embodiments, the ESL 119 is omitted.

Next, an Inter-Metal Dielectric (IMD) layer 121 is formed over the ESL119 (if formed) and over the ILD layer 117. The IMD layer 121 may beformed of a dielectric material such as silicon oxide, silicon nitride,silicon carbide, silicon oxynitride, or the like. In accordance withsome embodiments, the IMD layers 121 is formed of a low-k dielectricmaterial having a dielectric constant (k-value) lower than 3.0, such asabout 2.5, about 2.0, or even lower. The IMD layers 121 may comprise acarbon-containing low-k dielectric material, Hydrogen SilsesQuioxane(HSQ), MethylSilsesQuioxane (MSQ), or the like. The formation of the IMDlayer 121 may include depositing a porogen-containing dielectricmaterial over the ILD layer 117, and then performing a curing process todrive out the porogen, thereby forming the IMD layer 121 that is porous,as an example. Other suitable method may also be used to form the IMDlayer 121.

Next, in FIG. 2 , a tri-layered photoresist 126 comprising a topphotoresist layer 127, a middle layer 125, and a bottom anti-reflectivecoating (BARC) layer 123, is formed over the IMD layer 121. The BARClayer 123 of the tri-layered photoresist 126 may comprise an organic orinorganic material. The middle layer 125 may comprise silicon nitride,silicon oxynitride, or the like, that has an etch selectivity to the topphotoresist layer 127, such that the top photoresist layer 127 can beused as a mask layer to pattern the middle layer 125. The topphotoresist layer 127 may comprise a photosensitive material. Anysuitable deposition method, such as PVD, CVD, spin coating, the like, orcombinations thereof, may be used to form the tri-layered photoresist126.

Once the tri-layered photoresist 126 is formed, patterns 129 (e.g.,openings) are formed in the top photoresist layer 127. In an embodiment,the top photoresist layer 127 is patterned by exposing thephotosensitive material within the top photoresist layer 127 to apatterned energy source (e.g., light) through, e.g., a reticle. Theimpact of the energy will cause a chemical reaction in those parts ofthe photosensitive material that were impacted by the patterned energysource, thereby modifying the physical properties of the exposedportions of the photoresist such that the physical properties of theexposed portions of the top photoresist layer 127 are different from thephysical properties of the unexposed portions of the top photoresistlayer 127. The top photoresist layer 127 may then be developed with adeveloper to remove the exposed portion of the top photoresist layer 127or the unexposed portion of the top photoresist layer 127, depending on,e.g., whether a negative photosensitive material or a positivephotosensitive material is used for the top photoresist layer 127. Theremaining portions of the top photoresist layer 127 form a patternedphotoresist layer. Next, the patterns 129 in the top photoresist layer127 are extended through the middle layer 125 and the BARC layer 123,and are transferred to the IMD layer 121 using a suitable method, suchas one or more anisotropic etching processes.

FIG. 3 illustrates the cross-sectional view of the semiconductor device100 after the patterns of the top photoresist layer 127 are transferredto the IMD layer 121. In some embodiments, to transfer the patterns ofthe top photoresist layer 127, an anisotropic etching processes, such asan anisotropic plasma etching process, is performed. The anisotropicplasma etching process is a reactive-ion etching (RIE) process performedusing, e.g., a fluoride-containing etching gas, such as an etching gascomprising carbon hydrogen fluoride (C_(x)H_(y)F_(z) such as CHF₃), insome embodiments. As illustrated in FIG. 3 , after the anisotropicplasma etching process, openings 122 (may also be referred to as viaopenings) are formed in the IMD layer 121 and extend through the ESL 119(if formed), which openings 122 expose upper surfaces of the underlyingcontact plugs 113. The bottoms of the openings 122 in the example ofFIG. 3 are level with the upper surface of the ILD layer 117. FIG. 3illustrates two openings 122 as a non-limiting example. Other openingsmay be formed to expose the upper surfaces of other contact plugs 113,as skilled artisans readily appreciate. In some embodiments, thefluoride (F) in the etching gas may be implanted into the contact plugs113 during the anisotropic plasma etching process, and may be referredto as fluoride impurities in the contact plugs 113.

After the anisotropic plasma etching process, the tri-layeredphotoresist 126 is removed. In some embodiments, an ashing process (aplasma process) is performed to remove the tri-layered photoresist 126.The ashing process may be performed using a gas source comprising amixture of hydrogen gas (H₂) and a suitable inert gas, such as argon(Ar) or helium (He). In other words, the gas source used for the ashingprocess may be a mixture of H₂ and Ar, or a mixture of H₂ and He.

In an example embodiment, the ashing process is performed using a gassource comprising a mixture of H₂ and Ar. In some embodiments, the gassource (e.g., H₂ and Ar) is ignited into plasma for the ashing process.During the ashing process, H⁺ radicals generated from the gas sourceenter the contact plugs 113, collide with the fluoride impurities insidethe contact plugs 113 (that were generated by the plasma etching processdiscussed above to generate the openings 122), and drive the fluorideimpurities to the upper surfaces of the contact plugs 113. The fluorideimpurities driven to the upper surfaces of the contact plugs 113 reactwith the material (e.g., cobalt) of the contact plugs 113 to form, e.g.,cobalt fluoride (Co_(x)F_(y) such as CoF₂, CoF₃), in the illustratedembodiment. In other words, in the illustrated embodiment, the upperportions of the contact plugs 113 (e.g. portions proximate to the uppersurfaces of the contact plugs 113) are converted into cobalt fluoride.Notably, in the illustrated embodiment, the gas source for the ashingprocess is free of nitrogen. This is because a gas source comprisingnitrogen may convert the upper portions of the contact plugs 113 intocobalt nitride in the ashing process, and cobalt nitride could not beremoved by the subsequent wet cleaning process to form enlarged bottomportions for the openings 122.

In some embodiments, a mixing ratio R (e.g., volume ratio, or a flowrate ratio) between the inert gas (e.g., Ar) and H₂ in the gas source ofthe ashing process is equal to or smaller than 2 (e.g., R≤2). If themixing ratio R is larger than 2, the H⁺ radicals may be diluted toomuch, and may not be able to sufficiently collide with the fluorideimpurities and drive the impurities up to the upper surfaces of thecontact plugs 113. As will be discussed hereinafter with reference toFIG. 4A, the cobalt fluoride, which is formed by reaction between thefluoride impurities and cobalt at the upper surfaces of the contactplugs 113, will be removed by a subsequent wet cleaning process to formrecesses in the upper surfaces of the contact plugs 113, thereby formingenlarged bottom portions for the openings 122. Therefore, a mixing ratioR larger than 2 will negatively affect the efficiency of the H⁺ radicaltreatment of the contact plugs 113, and may prevent the recesses frombeing formed at the upper surfaces of the contact plugs 113.

In some embodiments, the process conditions of the ashing process istuned (e.g., adjusted) in accordance with the concentration of thefluoride in the contact plugs, in order to ensure sufficient treatmentof the contact plugs 113 by the ashing process. For example, the RFpower (e.g., for igniting the gas source into plasma) of the ashingprocess and/or the process time of the ashing process may be increasedfor high concentration (e.g., larger than 15 atomic percentage) offluoride in the contact plugs 113. In addition, or alternatively, thepressure of the ashing process may be decreased for high concentration(e.g., larger than 15 atomic percentage) of fluoride in the contactplugs 113. The increase plasma power, the increased process time, and/orthe decreased pressure help to enhance the probability of the H⁺radicals colliding with the fluoride impurities. To determine theconcentration of fluoride in the contact plug 113, a test wafer may beprocessed by the same anisotropic plasma etching process discussedabove, then an X-ray Photoelectron Spectroscopy (XPS) measurement isperformed to determine the concentration of the fluoride, as an example.

Next, in FIG. 4A, a wet cleaning process is performed to clean theopenings 122. In some embodiments, the wet cleaning process is performedin three successive steps. The first cleaning step is performed usingdeionized water (DIW), and may also be referred to as a DIW cleaningstep. The DIW cleaning step may be performed for, e.g., about 50seconds. Recall that the ashing process converts upper portions of thecontact plugs 113 into cobalt fluoride. In some embodiments, the upperportions of the contact plugs 113, which is cobalt fluoride, dissolve inthe DIW and are removed by the DIW cleaning step. Next, the secondcleaning step is performed using an acid, such as4-Methyl-2-(phenylamino)-1,3-thiazole-5-carboxylic acid (may also bereferred to as MS9600 acid). The second cleaning step may also bereferred to as an acid cleaning step. The acid cleaning step may beperformed for, e.g., 120 seconds. Next, the third cleaning step isperformed using isopropyl alcohol (IPA), and therefore, may also bereferred to as an IPA cleaning step. The IPA cleaning step may beperformed for, e.g., about 10 seconds to about 100 seconds. Note thatthe cleaning fluid and the by-products (if any) of each cleaning stepare removed after each cleaning step is finished and before the nextprocessing step (e.g., the next cleaning step) is performed, in someembodiments.

As discussed above, the wet cleaning process removes the upper portionsof the contact plugs 113, thereby enlarging the bottom portions of theopenings 122. As illustrated in FIG. 4A, the wet cleaning processrecesses the upper surfaces of the contact plugs 113, e.g., below theupper surface of the ILD layer 117. As a result, the enlarged bottomportions of the openings 122 extend into the contact plugs 113, and formundercuts under the IMD layer 121. In other words, the enlarged bottomportion of each of the openings 122 is wider than the opening 122, andtherefore, extends laterally beyond sidewalls of the IMD layer 121exposed by the opening 122. The enlarged bottom portion of each opening122 in FIG. 4A has curve sidewalls, and resembles portions of an ovalshape. Besides the illustrated shape, other shapes for the cross-sectionof the enlarged bottom portion of the opening 122 are also possible, andare fully intended to be included within the scope of the presentdisclosure.

In some embodiments, after the wet cleaning process, a plasma process isperformed to remove by-products (e.g., carbon-containing by-products)left by the wet cleaning process and/or the previous etching processes.The plasma process may be performed using a gas source comprising oxygen(O₂). The oxygen gas is ignited into plasma, and the plasma is suppliedto the opening 122 to remove, e.g., the carbon-containing by-products. Atemperature of the plasma process may be, e.g., 160° C.

FIG. 4B illustrates a zoomed-in view of an area 131 in FIG. 4A. Asillustrated in FIG. 4B, the enlarged bottom portion of the opening 122has a width C measured between, e.g., opposing sidewalls at a widestportion of the enlarged bottom portion, and a depth A between the uppersurface of the ILD layer 117 and a bottom of the opening 122. Inaddition, an undercut portion of the opening 122 has a width B. In otherwords, the undercut portion of the opening 122 extends beyond thesidewall 121S of the IMD layer 121 by a distance of B. In someembodiments, a ratio between A and C is between 0.28 and 0.32 (e.g.,0.28≤A/C≤0.32). In some embodiments, a ratio between A and B is between0.9 and 1.1 (e.g., 0.9≤A/B≤1.1). In some embodiments, a ratio between Band C is between 0.28 and 0.31 (e.g., 0.28≤B/C≤0.31). In someembodiments, a difference between A and B is less than 1 nm (e.g., A-B<1nm).

Next, in FIG. 5 , a conductive material 133, such as tungsten, is formedin the openings 122 to fill the openings 122. The conductive material133 may overfill the openings 122 and extend above the upper surface ofthe IMD layer 121. In some embodiments, the conductive material 133 isformed selectively over the contact plugs 113 using a suitable formationmethod (e.g., a slow ALD (SALD) process), such that the conductivematerial 133 is formed in a bottom-up fashion, thereby filling theopenings 122 better (e.g., with less or no voids) than, e.g., a bulkformation method, such as CVD or PVD. As illustrated in FIG. 5 , theconductive material 133 fills the undercut portions of the enlargedbottom portions of the opening 122, and therefore, forms vias 133 withenlarged bottom portions. The enlarged bottom portion of each of thevias 133 is wider than an overlying portion (e.g., a portion betweenopposing sidewalls of the IMD layer 121 proximate to the lower surfaceof the IMD layer 121) of the vias 133. In some embodiments, the enlargedbottom portion of each via 133 extends laterally beyond opposingsidewalls of the IMD layer 121 facing the via 133.

Next, in FIG. 6 , a barrier layer 135 is formed over the upper surfaceof the IMD layer 121. The barrier layer 135 may comprise titanium,titanium nitride, or the like, formed by a suitable formation methodsuch as PVD, CVD, ALD, or the like. Next, a conductive material 137 isformed over the barrier layer 135. In the illustrated embodiment, theconductive material 137 is the same material (e.g., tungsten) as theconductive material 133, but formed of a different formation method. Forexample, instead of the selective, bottom-up formation method for theconductive material 133, the conductive material 137 is formed of a bulkformation method such as CVD, PVD, or sputtering. The bulk formationmethod may be used to reduce the process time for forming the conductivematerial 137.

Next, in FIG. 7 , a planarization process, such as CMP, is performed toremove the conductive material 137 and the barrier layer 135 from theupper surface of the IMD layer 121, and to achieve a coplanar uppersurface between the IMD layer 121 and the vias 133. In some embodiments,the interface between the conductive material 137 and the barrier layer135 serves as an etch-stop mark for the CMP process, such that the CMPprocess can be stopped after the upper surface of the IMD layer 121 isexposed.

As feature size continues to shrink in advanced processing nodes, itbecomes increasingly difficult to form vias 133 due to the high aspectratio of the via openings. In order to make it easier to fill the viaopenings, the vias 133 may be formed without a barrier layer or anadhesion layer. Therefore, adhesion between the vias 133 and theirsurrounding layers (e.g., the IMD layer 121 and the ESL 119) maydegrade, and tiny cracks may exist between, e.g., the vias 133 and theIMD layer 121/ESL 119. During the CMP process, the slurry used in theCMP process (may also be referred to as CMP slurry) may seep downthrough the cracks and reach the contact plugs 113. The slurry may havea high etch selectivity (e.g., having a high etch rate) for the material(e.g., cobalt) of the contact plugs 113, and therefore, may cause theupper surfaces of the contact plugs 113 to recess, thereby causingunreliable electrical connection between the vias 133 and the underlyingcontact plugs 113. The disclosed method, by enlarging the bottom portionof the via openings, allows vias 133 with enlarged bottom portions to beformed. The enlarged bottom portions of the vias 133 form a better sealaround the cracks, e.g., at the bottom surface of the ESL 119 to reducethe amount of CMP slurry that can seep through the ESL 119. In addition,the enlarged bottom portions of the vias 133 may also catch a smallamount of CMP slurry that does seep through the ESL 119. Therefore,during the CMP process, the CMP slurry may seep down through the cracks(e.g., along the sidewalls of the vias 133) between the vias 133 and theIMD layer 121, but most of the CMP slurry is either stopped in the ESL119 by the enlarged bottom portions of the vias 133, or is caught by theenlarged bottom portion of the vias 133. As a result, the amount of CMPslurry reaching the contact plug 113 is reduced or eliminated, therebyreducing or preventing recessing of the contact plugs 113. Anotheradvantage is reduced electrical resistance for the vias 133, due to theenlarged bottom portions, which may also reduce the RC delay of thesemiconductor device formed.

Next, in FIG. 8 , a dielectric layer 139 is formed over the IMD layer121. The dielectric layer 139 may be formed of a same or similarmaterial as the IMD layer 121 by the same or similar formation method,thus details are not repeated. Next, vias 141 are formed in thedielectric layer 139 over and electrically couple to the underlying vias133. The vias 141 may be formed by forming via openings in thedielectric layer 139 to expose the underlying vias 133, filling the viaopenings with a conductive material, and performing a CMP process afterfilling the via openings. In the illustrated embodiment, the vias 141are formed of a same material (e.g., tungsten) as the vias 133. In theexample of FIG. 8 , the vias 141 do not have the enlarged bottomportions as the vias 133, because the slurry used for the CMP processhas a high etch rate for the material (e.g., cobalt) of the contactplugs 113, but has a low etch rate for the material (e.g., tungsten) ofthe vias 133. In other embodiments, the vias 141 may be formed to haveenlarged bottom portions same as or similar to the enlarged bottomportions of the vias 133.

Additional processing steps may follow the processing of FIG. 8 tocomplete the fabrication of the semiconductor device 100. For example,vias 143 may be formed over and electrically coupled to the gatestructures 109. The vias 143 may be formed to extend through the IMDlayer 121 and the dielectric layer 139 to be in contact (e.g., physicalcontact) with the cap layer 115 (e.g., tungsten) of the gate structures109. Additional layers of dielectric layers may be formed over thedielectric layer 139, and conductive features (e.g., conductive linesand vias) may be formed in the additional layers of dielectric layers toform the interconnect structures of the semiconductor device 100, whichinterconnect structures electrically interconnect the underlyingelectrical components to form functional circuits. Details are notdiscussed here.

Variations to the disclosed embodiments are possible and are fullyintended to be included within the scope of the present disclosure. Forexample, while the disclosed method is described in the context offorming a via over a source/drain contact plug, the spirit of thedisclosure may be applied to form conductive features (e.g., vias) inother dielectric layers of the semiconductor device over other types ofconductive features (e.g., conductive lines). As another example,besides the vias 133, other vias (e.g., 141) formed over the vias 133may also be formed with enlarged bottom portions.

Embodiments may achieve advantages. The disclosed method form viaopenings with enlarged bottom portions, which allows vias (e.g., 133)with enlarged bottom portions to be formed. The enlarged bottom portionsof the vias reduce the amount of CMP slurry that seeps down throughcracks between the vias and the dielectric layer around the vias,thereby reducing the recessing of the conductive features (e.g., contactplugs 113) underlying the vias and improving the reliability of theelectrical connection between the vias and the underlying conductivefeatures. Additional advantage includes reduced electrical resistancefor the vias and reduced RC delay for the device formed.

FIG. 9 illustrates a flow chart of a method of fabricating asemiconductor structure, in accordance with some embodiments. It shouldbe understood that the embodiment method shown in FIG. 9 is merely anexample of many possible embodiment methods. One of ordinary skill inthe art would recognize many variations, alternatives, andmodifications. For example, various steps as illustrated in FIG. 9 maybe added, removed, replaced, rearranged, or repeated.

Referring to FIG. 9 , at block 1010, a first conductive feature isformed in a first dielectric layer disposed over a substrate. At block1020, a second dielectric layer is formed over the first dielectriclayer. At block 1030, the second dielectric layer is etched using apatterned mask layer to form an opening in the second dielectric layer,wherein the opening exposes the first conductive feature. At block 1040,an ashing process is performed to remove the patterned mask layer afterthe etching. At block 1050, the opening is cleaned by a wet cleaningprocess after the ashing process, wherein the wet cleaning processenlarges a bottom portion of the opening. At block 1060, the opening isfilled with a first electrically conductive material.

In accordance with an embodiment of the present disclosure, a method offorming a semiconductor device includes: forming a first conductivefeature in a first dielectric layer disposed over a substrate; forming asecond dielectric layer over the first dielectric layer; etching thesecond dielectric layer using a patterned mask layer to form an openingin the second dielectric layer, wherein the opening exposes the firstconductive feature; performing an ashing process to remove the patternedmask layer after the etching; wet cleaning the opening after the ashingprocess, wherein the wet cleaning enlarges a bottom portion of theopening; and filling the opening with a first electrically conductivematerial. In an embodiment, the wet cleaning removes a top portion ofthe first conductive feature distal from the substrate and formsundercuts under the second dielectric layer. In an embodiment, after thewet cleaning, the bottom portion of the opening extends vertically intothe first conductive feature and extends laterally beyond sidewalls ofthe second dielectric layer exposed by the opening. In an embodiment,etching the second dielectric layer comprises etching the seconddielectric layer by performing a first plasma process, whereinperforming the ashing process comprises performing a second plasmaprocess different from the first plasma process. In an embodiment, thefirst plasma process is performed using a gas source comprisingfluoride, wherein the second plasma process is performed using a gassource comprising hydrogen. In an embodiment, the fluoride is implantedin the first conductive feature by the first plasma process, wherein themethod further comprises adjusting an RF power of the second plasmaprocess, a process time of the second plasma process, or a pressure ofthe second plasma process in accordance with a concentration of fluoridein the first conductive feature. In an embodiment, the wet cleaningcomprises: performing a first wet cleaning step using deionized water;performing a second wet cleaning step using an acid; and performing athird wet cleaning step using isopropyl alcohol. In an embodiment, thegas source of the first plasma process comprises C_(x)H_(y)F_(z), andthe gas source of the second plasma process comprises a mixture of H₂and an inert gas, wherein the inert gas is Ar or He. In an embodiment, amixing ratio between the inert gas and the H₂ in the gas source of thesecond plasma process is equal to or smaller than 2. In an embodiment,the method further includes, after filling the opening: forming abarrier layer over an upper surface of the second dielectric layerdistal from the substrate; forming a second electrically conductivematerial over the barrier layer, wherein the first electricallyconductive material and the second electrically conductive materialcomprise a same material but are formed using different formationmethods; and performing a planarization process to remove the barrierlayer and the second electrically conductive material. In an embodiment,the method further includes, after the planarization process: forming athird dielectric layer over the second dielectric layer; and forming avia in the third dielectric layer over and electrically coupled to thefirst electrically conductive material.

In accordance with an embodiment of the present disclosure, a method offorming a semiconductor device, the method includes: forming a firstconductive feature in a first dielectric layer; forming a seconddielectric layer over the first dielectric layer; forming an opening inthe second dielectric layer to exposes an upper surface of the firstconductive feature, wherein forming the opening comprises performing ananisotropic plasma etching process; after forming the opening, treatingthe first conductive feature with a plasma process different from theanisotropic plasma etching process; and after the plasma process,cleaning the opening using deionized water, wherein the cleaningenlarges a bottom portion of the opening. In an embodiment, the cleaningrecesses an upper surface of the first conductive feature facing thesecond dielectric layer and forms undercuts under the second dielectriclayer. In an embodiment, the method further includes filling the openingwith an electrically conductive material after the cleaning, wherein theelectrically conductive material fills the undercuts. In an embodiment,the anisotropic plasma etching process is performed using a first gassource comprising C_(x)H_(y)F_(z), and wherein the plasma process isperformed using a second gas source comprising a mixture of H₂ and Ar ora mixture of H₂ and He. In an embodiment, the first conductive featureis formed of cobalt.

In accordance with an embodiment of the present disclosure, a method offorming a semiconductor device includes: forming a first conductivefeature in a first dielectric layer disposed over a substrate; forming asecond dielectric layer over the first dielectric layer; etching thesecond dielectric layer to form an opening in the second dielectriclayer, wherein the first conductive feature is exposed at a bottom ofthe opening; after the etching, treating the first conductive featurewith a plasma process, wherein the plasma process converts an upperportion of the first conductive feature into a first material; after theplasma process, cleaning the opening by a wet cleaning process, whereinthe wet cleaning process comprises a wet cleaning step using deionizedwater, wherein the deionized water removes the first material andenlarges the bottom of the opening; and after the cleaning, filling theopening with an electrically conductive material. In an embodiment,etching the second dielectric layer comprises performing a plasmaetching process using an etching gas comprising fluoride, wherein theplasma process is performed using a gas source comprising hydrogen. Inan embodiment, the enlarged bottom of the opening extends into the firstconductive feature and includes undercuts under the second dielectriclayer, wherein after filling the opening, the electrically conductivematerial fills the undercuts. In an embodiment, the first conductivefeature is formed of cobalt, and the electrically conductive material istungsten.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of forming a semiconductor device, themethod comprising: forming a first conductive feature in a firstdielectric layer; forming a second dielectric layer over the firstdielectric layer; forming an opening in the second dielectric layer toexposes an upper surface of the first conductive feature, whereinforming the opening comprises performing an anisotropic plasma etchingprocess; after forming the opening, treating the first conductivefeature with a plasma process different from the anisotropic plasmaetching process; and after the plasma process, cleaning the openingusing deionized water, wherein the cleaning enlarges a bottom portion ofthe opening.
 2. The method of claim 1, wherein the cleaning recesses theupper surface of the first conductive feature, and forms undercuts underthe second dielectric layer, wherein the undercuts extend beyondsidewalls of the second dielectric layer exposed by the opening.
 3. Themethod of claim 2, further comprising, after the cleaning, forming a viaby filling the opening with an electrically conductive material.
 4. Themethod of claim 3, wherein the first conductive feature is formed ofanother electrically conductive material different from the electricallyconductive material of the via.
 5. The method of claim 3, wherein thefirst conductive feature is formed of cobalt, and the via is formed oftungsten.
 6. The method of claim 5, wherein the anisotropic plasmaetching process is performed using a first gas source comprisingC_(x)H_(y)F_(z), and the plasma process is performed using a second gassource comprising a mixture of H₂ and an inert gas.
 7. The method ofclaim 6, wherein the second gas source is free of nitrogen.
 8. A methodof forming a semiconductor device, the method comprising: forming afirst conductive feature in a first dielectric layer disposed over asubstrate; forming a second dielectric layer over the first dielectriclayer; etching the second dielectric layer to form an opening in thesecond dielectric layer, wherein the first conductive feature is exposedat a bottom of the opening; after the etching, treating the firstconductive feature with a plasma process, wherein the plasma processconverts an upper portion of the first conductive feature into a firstmaterial; after the plasma process, cleaning the opening by a wetcleaning process, wherein the wet cleaning process comprises a wetcleaning step using deionized water, wherein the deionized water removesthe first material and enlarges the bottom of the opening; and after thecleaning, filling the opening with an electrically conductive material.9. The method of claim 8, wherein etching the second dielectric layercomprises performing a plasma etching process using a first gas sourcecomprising fluoride.
 10. The method of claim 9, wherein treating thefirst conductive feature comprises performing the plasma process using asecond gas source comprising hydrogen.
 11. The method of claim 10,wherein the first conductive feature is formed of cobalt, and theelectrically conductive material is tungsten.
 12. The method of claim 8,wherein the enlarged bottom of the opening extends into the firstconductive feature and includes undercuts under the second dielectriclayer.
 13. The method of claim 12, wherein a widest portion of theenlarged bottom of the opening is in the first dielectric layer, whereina width of the widest portion of the enlarged bottom of the opening islarger than a distance between opposing sidewalls of the seconddielectric layer exposed by the opening.
 14. The method of claim 8,wherein filling the opening with the electrically conductive materialforms a first via, wherein the first via has an upper portion in thesecond dielectric layer and has an enlarged bottom portion in the firstdielectric layer, wherein the method further comprises, after fillingthe opening: forming a third dielectric layer over the second dielectriclayer; and forming a second via in the third dielectric layer, whereinthe second via is formed over, and connected to, the first via, whereinthe second via is free of an enlarged bottom portion.
 15. The method ofclaim 14, further comprising, after filling the opening and beforeforming the third dielectric layer: forming a barrier layer over thesecond dielectric layer; forming a conductive layer over the barrierlayer, wherein the conductive layer is formed using the electricallyconductive material; and after forming the barrier layer and theconductive layer, performing a planarization process to remove theconductive layer and the barrier layer.
 16. The method of claim 15,wherein the electrically conductive material used for filling theopening is formed by a first formation method, and the electricallyconductive material used for forming the conductive layer is formed by asecond formation method different from the first formation method.
 17. Amethod of forming a semiconductor device, the method comprising: forminga first conductive feature in a first dielectric layer disposed over asubstrate, wherein the first conductive feature is a contact plugcomprising cobalt; forming a second dielectric layer over the firstdielectric layer; etching the second dielectric layer using a first gassource comprising fluoride to form an opening in the second dielectriclayer, wherein the first conductive feature is exposed at a bottom ofthe opening; after the etching, treating the first conductive featurewith a plasma process using a second gas source comprising hydrogen,wherein the plasma process converts an upper portion of the firstconductive feature into a first material; after the plasma process,cleaning the opening by a wet cleaning process, wherein the wet cleaningprocess comprises a wet cleaning step using deionized water, wherein thedeionized water removes the first material and enlarges the bottom ofthe opening; and after the cleaning, filling the opening with anelectrically conductive material.
 18. The method of claim 17, whereinetching the second dielectric layer comprises performing an anisotropicplasma etching process using the first gas source comprising fluoride,wherein the anisotropic plasma etching process implants fluoride in theupper portion of the first conductive feature.
 19. The method of claim18, wherein the first material is cobalt fluoride, wherein the wetcleaning process further comprises, after the wet cleaning step usingdeionized water: a second cleaning step performed using a acid; and athird cleaning step performed using isopropyl alcohol.
 20. The method ofclaim 19, further comprising, after the wet cleaning process and beforefilling the opening, performing another plasma process to cleanby-products of the wet cleaning process, wherein the another plasmprocess is performed using a third gas source comprising oxygen.